These things took place in Bethany beyond the Jordan, where John was baptizing. The next day he saw Yeshua coming to him and said "Behold the Lamb of GOD who takes away the sin of the world." John 1:28, 29
In the time of Yeshua, sacrificial lambs were kept within the confines of the Temple. Should you be required to offer a lamb as a sacrifice, you would be led to the pens containing the lambs chosen to be sacrificed. With the help of a Temple Priest, your selection would be made. The Priest would then point to a chosen lamb and proclaim "Behold the Lamb of GOD." That particular lamb would then be slain as a sacrificial offering to atone for your sins.
We are reminded that John the Baptist was a Temple Priest, as was his father Zechariah, as told in Luke 1:5. It is not surprising that the words "Behold the Lamb of GOD" would leap from his mouth upon seeing Yeshua of Nazareth coming to be baptized. John knew in his heart that this Lamb of GOD would be slain, not for the sin of one person, but for the sins of the world. "Behold the Lamb of GOD who takes away the sin of the world."